Combining Freestanding Ferroelectric Perovskite Oxides with Two-Dimensional Semiconductors for High Performance Transistors

We demonstrate the fabrication of field-effect transistors based on single-layer MoS2 and a thin layer of BaTiO3 (BTO) dielectric, isolated from its parent epitaxial template substrate. Thin BTO provides an ultrahigh-κ gate dielectric effectively screening Coulomb scattering centers. These devices show mobilities substantially larger than those obtained with standard SiO2 dielectrics and comparable with values obtained with hexagonal boron nitride, a dielectric employed for fabrication of high-performance two-dimensional (2D) based devices. Moreover, the ferroelectric character of BTO induces a robust hysteresis of the current vs gate voltage characteristics, attributed to its polarization switching. This hysteresis is strongly suppressed when the device is warmed up above the tetragonal-to-cubic transition temperature of BTO that leads to a ferroelectric-to-paraelectric transition. This hysteretic behavior is attractive for applications in memory storage devices. Our results open the door to the integration of a large family of complex oxides exhibiting strongly correlated physics in 2D-based devices.

T he development of methods and tools to deterministically transfer two-dimensional (2D) materials in 2010 1−3 opened the door to fabricate artificial matter by simply stacking 2D materials on top of each other, without being constrained by the rigid limitations of epitaxial growth: lattice parameter matching, complexity of the technique, and price of the required tools. 4 Since then, this emerging field of research, the so-called van der Waals heterostructures, 5−9 has continued to grow at a tremendous pace. Novel physical phenomena 10 and device concepts 11−13 have been achieved by these straightforward methods of combining 2D materials.
Apart from vertically stacking 2D materials, a great deal of interest has been focused on using these deterministic transfer methods to combine/integrate 2D materials with other families or classes of materials, including epitaxially grown complex oxides, in mixed-dimensional van der Waals heterostructures. 14−16 Within this context, the isolation of 2D non-van der Waals materials opens up an interesting avenue. 17 −21 In particular, the recent fabrication of freestanding layers of complex transition metal oxides of just a few unit cells in thickness 22−28 provides a new research arena as this family of materials presents a whole plethora of strongly correlated physics that is scarcely present in only a handful of van der Waals 2D materials. The integration of these freestanding complex oxides with 2D materials, however, was not reported until very recently by two research groups in parallel with preparation of our work. In these two recent works 29,30 SrTiO 3 (STO) thin layers are isolated freestanding and used as high-κ dielectrics for field-effect transistors based on 2D materials. Yang et al. show that in devices utilizing a freestanding layer of STO, ON/OFF ratios of 10 8 and subthreshold swings of 66 mV/dec can be achieved. 30 Huang et al. show that the STO layer is exceptional for use as a dielectric layer because it possesses a subone-nanometer capacitance equivalent thickness with a low leakage current (less than 10 −2 A·cm −2 at 2.5 MV·cm −1 ). 29 Prior to the isolation of freestanding layers of complex oxides, 2D materials were already integrated with complex oxide dielectrics epitaxially grown onto 3D oxide substrates to improve and add new functionalities to 2D-based devices. 31 These previous works show a glimpse of the potential of combining van der Waals 2D materials with freestanding complex oxides. Particularly appealing in this field is the use of ferroelectric oxides, presenting a spontaneous polarization that can be reversed by a suitable electric field in the opposite direction. 32 This polarization influences the electronic properties of the 2D material-oxide interface, and its switchable character provides an additional tuning parameter for device operation. Memristive properties have, in this way, been added to 2D-based field effect transistors and tunnel junctions. 33−42 As already achieved with the dielectric STO, 29,30 any step forward in facilitating the stackable aspect of these ferroelectric components would immediately expand their potential applications, using the van der Waals integration strategy to fabricate novel hybrid 2D heterostructures.
In this work, we isolate freestanding crystalline thin layers of BTO, one of the most widely researched ferroelectric materials, and integrate them for use as a ferroelectric gate dielectric in field effect transistors based on single-layer MoS 2 semiconducting channels. We found that BTO acts as a very effective high-κ dielectric, screening out Coulomb scattering centers and yielding larger mobility values than those obtained  with standard SiO 2 dielectric. 43 In comparison with the use of 2D ferroelectric van der Waals materials such as CuInP 2 S 6 as a dielectric layer, 44 BTO offers a much higher ferroelectric moment 45 (26 μC/cm 2 vs 3.8 μC/cm 2 ), electric permittivity 46 (∼4000 vs ∼50) at room temperature, larger band gap 47−49 (∼3.3 eV vs 2.6 eV) and higher Curie temperature 45 (420 K vs 320 K). Moreover, the current versus gate voltage traces of the MoS 2 field-effect transistors based on BTO dielectric present a very robust hysteresis, characteristic of the ferroelectric nature of BTO. This adds an additional characteristic beyond exceptional device properties when compared with recent results utilizing STO. 29,30 This hysteretic behavior can be used to fabricate low energy consumption transistors or memory storage devices. We compare the performance of the fabricated BTO-based devices with that obtained in MoS 2 transistors fabricated with hexagonal boron nitride (hBN) flakes of similar thickness finding comparable mobility values, indicating that the ultrahigh dielectric constant of BTO effectively screens out the charge impurities.
The BTO layers, 15−50 nm thick, were epitaxially grown by high pressure (pure oxygen) sputtering on (001) STO substrates covered by (15 nm) La 0.7 Sr 0.3 MnO 3 (LSMO) epitaxial buffer layers. The LSMO layer is used as a sacrificial layer to enable the BTO delamination. We take advantage of the fact that a HCl and KI based solution etches LSMO very effectively without damaging the BTO film to release the BTO from the parent substrate (see Materials and Methods). Prior to the LSMO etching, a Gel-Film square (Gel-Film WF 4× 6.0 mil by Gel-Pac) is adhered to the BTO surface to facilitate the handling of the BTO once it detaches from the parent substrate. Figure 1a illustrates the steps employed to delaminate the BTO layer from its parent substrate and Figure  1b shows an optical image of a Gel-Film with a ∼2 by 2 mm 2 BTO layer (25 nm thick), obtained after the release process.
The structure and composition of as-isolated BTO freestanding layers were characterized by high-resolution scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS). Figure 2a shows a high-angle annular dark-field (HAADF) image of an as-isolated freestanding BTO layer released from the substrate and transferred onto a standard Si 3 N 4 holey membrane. Details about sample preparation can be found in the Materials and Methods. Atomic resolution HAADF characterization of the same flake along the [001] direction evidence a highly ordered perovskite structure, as shown in the image of Figure 2b. Occasionally, stacking faults or other growth defects were observed in the images. Quantitative analysis of high-resolution STEM images (see the Supporting Information Figure S1) allows for the determination of the in-plane lattice parameters of the freestanding BTO layer. We obtain 4.10 and 4.13 Å for the in-plane lattice parameters along the high symmetry (x and y) directions of the tetragonal ferroelectric structure. These values are somewhat larger than the expected bulk in-plane values (4 Å) which could result from an in-plane polarization in the freestanding BTO layers. EELS spectra at the Ti-L 2,3 and O-K edges were measured over the region shown in Figure 2b (see Figure 2c). Ti-L 2,3 edge fine structure shows a clear splitting between t 2g and e g peaks, characteristic of a 4+ Ti oxidation state, as expected from the BTO bulk stoichiometry. Quantification of the Ti oxidation state was carried out using a multiple linear least-squares fitting (MLLS) procedure to LaTiO 3 (Ti 3+ ) and BaTiO 3 bulk (Ti 4+ ) reference spectra and averaged values of 3.99 ± 0.01 were obtained. 50 We additionally characterized the freestanding BTO flakes using optical microscopy, microreflectance, Raman spectroscopy, photoluminescence spectroscopy (see Supporting Information Figures S2, S3 and S4). Furthermore, X-ray diffraction and reflectivity has been used to determine the out-of-plane lattice parameter of the freestanding BTO films (finding a value of c = 4.00 Å, indicating relaxation of the crystal lattice as compared to the epitaxial BTO layers), see Supporting Information Figures S5 and S6.
Functional characterization of the as-isolated BTO layers was performed by Piezoresponse Force microscopy (PFM) to assess the preservation of the ferroelectric character and its switchable nature with applied electric fields. For this purpose, the BTO layers were transferred to Au-coated SiO 2 /Si substrates (bottom electrode) while the conductive AFMprobe is used as the mobile top electrode. Figure 2d shows a PFM phase map for a BTO sample of similar characteristics as those used for device fabrication subjected to domain writing by patterning a box-in-box design with opposite tip voltages (+5 and −5 V) above the coercive voltages. This result demonstrates that the ferroelectricity in the BTO films survives even in freestanding (peeled off from the parent epitaxy template substrate) form. Figure 2e shows a representative hysteresis loop on a selected location of the BTO flake. The reversal of the phase signal and the butterfly like amplitude loop are the footprint of the 180°polarization switching. The phase loop is quite symmetric with respect to the sign of the applied voltage. From similar cycles acquired at different locations of the same flake (see Supporting Information Figure  S7) we obtain coercive voltages of 3.1 ± 0.6 V and −2.4 ± 0.3 V for the positive and negative regions, respectively. Asymmetric loops had also been measured in some of the flakes of different thickness (Supporting Information Figure  S7), presenting a shift to the negative applied tip voltages. This negative imprint corresponds to a preferred down-polarization state of the BTO, which was also found for similar heterostructures using nonfreestanding BTO thin films. 51 In the following, we show that the robust ferroelectric state of the single crystalline freestanding BTO layers can be used in combination with single-layer MoS 2 layers to engineer a field effect device. For the device fabrication, a piece of Nitto SPV 224 tape is adhered onto the Gel-Film substrate with the thin BTO layer and peeled off suddenly. This leads to the transfer of smaller area (30 × 30 μm 2 , approx.) "flakes" of BTO that have more suitable dimension for our device geometry. We have explored two different approaches to deterministically transfer the BTO flakes that will be used as dielectric between two gold electrodes prepatterned on a SiO 2 (285 nm)/Si substrate (Figures 3a and 3b): a deterministic transfer based on the use of nail polish polymer and one based on deterministic transfer directly from the Nitto-tape (see Materials and Methods). We note that although it is possible to create devices directly on BTO on an epitaxial growth substrate, our freestanding transfer on to SiO 2 will allow development toward for more complex device architectures (dual-gated devices) and easier extraction of device properties, such as mobilities calculated below.
The semiconductor channel is then fabricated by transferring a single-layer MoS 2 flake onto the BTO flake and bridging the two prepatterned gold electrodes leading to a van der Waals mediated electrical contact (Figure 3c). Briefly, a bulk natural molybdenite mineral (Molly Hill Mine, Quebec, Canada) 52 was cleaved with Nitto SPV 224 tape. Then the tape containing the cleaved micro crystals was put in contact to a Gel-Film surface and peeled off gently to leave some atomically thin flakes on the Gel-Film substrate. The surface of the Gel-Film is then inspected under an optical microscope (Motic BA 310 MET-T) in transmission mode to identify suitable single-layers of MoS 2 (see inset in Figure 3c). Their thickness is first assessed by their apparent transmittance 53 and then double-checked by microreflectance spectroscopy to verify single-layer flakes. 54,55 Once a suitable single-layer MoS 2 flake is identified it is transferred with the all-dry viscoelastic deterministic placement method. 56,57 Note that, when the single layer MoS 2 flake is connected to a multilayered portion, a laser trimming process was carried out to ensure that all the electrical transport occurs through the single-layer MoS 2 (see Materials and Methods and Supporting Information Figures S8 and S9). 58 The devices are electrically characterized under high-vacuum conditions (∼10 −6 mbar) in a home-built vacuum probe station system. 59 An in situ annealing at 200°C for 2 h at highvacuum (∼10 −4 to 10 −5 mbar) was performed prior to electrical transport measurements to improve the gold-MoS 2 contact and to remove atmospheric adsorbates. Figure 4a shows source-drain current, at a fixed V bias = 5 V, while the gate voltage is swept from −50 V to +50 V (at 1 V/s rate). See the Supporting Information Figures S10 and S11 for measurements on devices with hBN and BTO dielectrics at different gate sweeping rates (from 0.1 V/s to 10 V/s). The inset in Figure 4a shows a collection of I ds vs V bias curves (IVs hereafter) acquired at different gate voltages. From the slope of the I sd vs V g (IV g hereafter) shown in Figure 3a one can extract the field-effect mobility of the transistor 60 where ∂I sd /∂V g is the slope of the IV g trace, L, and W are the length and width of the semiconductor channel, respectively, C is the capacitance per unit area between channel and backgate electrode and V sd is the source-drain bias. In our geometry, the capacitance can be calculated as two parallel plate capacitors in series, one with SiO 2 and another one with BTO as dielectrics where C X = ε 0 ·ε x /d x with ε 0 the vacuum electrical permittivity and ε x and d x the relative permittivity and the thickness of medium "x" (SiO 2 or BTO) respectively. Because of the very high ε r expected for BTO (∼4000), 61−63 in this geometry the capacitance will be strongly dominated by the SiO 2 contribution. This is advantageous to extract accurate fieldeffect mobility values even in the case of not knowing the exact dielectric value of the BTO ultrathin layer. We have fabricated a parallel plate capacitor using a freestanding BTO layer as dielectric to experimentally determine its dielectric constant (see Supporting Information Figure S12) finding a value of ε BTO = 4700 ± 500, in good agreement with the values reported for bulk BTO at room temperature. 61−63 In Figure 4a we note that the additional structure in the IV g curve (double hump feature) is a manifestation of the ferroelectric switching of the BTO layer. Similar features have been observed in devices with a ferroelectric layer. 35,37,64,65 We fabricated and tested seven devices integrating a BTO dielectric finding two-terminal field-effect mobility values ranging from 5 to 70 cm 2 /(V s) (see the Supporting Information, Table S1 for a summary of the characteristics of all devices measured). Those values are substantially larger than those typically found on devices fabricated using the same protocol but with standard SiO 2 dielectric (see Figure 4b) which are in the 0.1−10 cm 2 /(V s) range. Moreover, in order to put those large mobility values into context, we directly compare the performance of the MoS 2 transistors integrating BTO dielectric with that of a device fabricated following the same steps but replacing the BTO flake by a hBN of similar thickness (Figure 4c). Note than hBN is used in 2D-based devices because it is considered as an almost ideal dielectric substrate due to its atomically flat surface and very low density of Coulomb scattering centers. 1,66 Interestingly, the mobility of the BTO based devices is comparable to that of the hBN ones (1 to 80 cm 2 /(V s) for 4 measured devices) which we attribute to the very effective screening of the charged impurities due to the very high dielectric constant of BTO. 67,68 Another important figure-of-merit of field-effect transistors is the current ON-OFF ratio, I ON/OFF , which provides information about how effectively the transistor can be switched OFF. Most of the BTO based devices show I ON/OFF in the 10 5 to 10 7 range, comparable with the SiO 2 (∼10 6 ) or the hBN based transistors (∼10 5 to 10 8 ). The two BTO devices fabricated by nail polish transfer showed a poor switching performance (I ON/OFF of ∼10) which we attribute to chemical doping of the MoS 2 by the presence of traces of traces of nail polish that could not be removed with our current cleaning protocol. We are working on improving the cleaning process to avoid this issue, as nail polish transfer makes device assembly more straightforward than the deterministic Nitto tape based transfer, but that lays beyond the scope of the current work.
Given the ferroelectric character of BTO, it is important to study the transconductance curves both in forward and backward sweeps to draw our attention to the hysteresis. Figure 5a−c compares the current vs gate voltage curves acquired while sweeping the gate from −50 Vto +50 V and back to −50 V for MoS 2 field-effect transistors with BTO, SiO 2 and hBN dielectrics. The hysteresis of the BTO based devices ranges from 16 to 44 V, substantially larger than that of devices with nonferroelectric dielectrics where the hysteresis typically

Nano Letters pubs.acs.org/NanoLett
Letter arises from the polarizability of adsorbed or trapped molecules on the surface or interfaces. Given the large difference in dielectric constant between the BTO layer (∼4700) and the SiO 2 (∼3.9) one can expect that most of the gate voltage difference will occur across the SiO 2 dielectric and thus the BTO film will not be in a fully saturated ferroelectric poled state. This, in combination with the presence of adsorbate traps at the different interfaces (see below) and the intrinsic inplane polarization of the freestanding BTO flakes (see Figure  S1), will lead to a depolarization of the devices over time. We address the reader to the Supporting Information Figure S13 for a data set of retention time measurements performed at different temperatures. While ferroelectric-induced modulation in transistors is expected to induce counterclockwise hysteresis loops, 69,70 a clockwise hysteresis is observed in Figure 5a. This inversion is actually a relatively common result already reported in 2D FETs with ferroelectric dielectrics, first with a graphene channel 36,40,71 but also in transition metal dichalcogenide based transistors (including MoS 2 ). 39,65,70,72 Most studies agree that there is a subtle interplay between the ferroelectric polarization and interfacial phenomena, attributing the unexpected clockwise hysteresis to the interfacial charge screening of ferroelectric polarization. Ultrathin oxide ferroelectrics like BTO are typically oxygen deficient, and the oxygen vacancies accumulate at the surface. 73 The coupling of these oxygen vacancies to the ferroelectric polarization to compensate polarization charges could be responsible for polarization-assisted charge trapping at the BTO/MoS 2 interface. 71,72 Moreover, although our devices have been annealed in high-vacuum (200°C for 2 h.) prior to electrical characterization, remaining adsorbates trapped at the interface between the semiconducting 2D material and the dielectric, that could potentially contribute to screening the ferroelectric polarization, would explain the clockwise hysteresis of the transfer curves.
To get further insight on the role of the BTO ferroelectric character on the electrical behavior of the devices, we performed temperature dependent analysis of the current vs gate voltage curves, to follow the evolution of the hysteresis. These measurements can help to identify the origin of the hysteresis as shown for SiO 2 -gated devices. 74 In our BTObased heterostructures, increasing temperature brings an additional interesting phenomena: the ferroelectric to paraelectric transition of the BTO (above 120°C for bulk crystals). 75,76 If the large hysteresis measured in BTO-based devices is related with the polarization of BTO, a change is expected when measuring above the transition temperature. Figures 5d and 5e present the transfer curves and the hysteresis (calculated as the difference between voltages at I ON /2 for the forward and backward sweeps in the current vs gate voltage curves, see arrows in Figure 5a to 5c, as a function of temperature. From 70°C there is a decreasing trend of the hysteresis that almost vanishes above 150°C, in good agreement with the expected ferroelectric to paraelectric transition. The larger hysteresis values are recovered upon decreasing the temperature back to room temperature. This behavior clearly differs from that observed in the hBN and SiO 2 -gated devices, which show an overall increase with temperature, frequently related to thermally activated oxide/ adsorbate traps (close to the interface) that can capture and release electrons from MoS 2 . 74 We address the readers to the Supporting Information Figure S14 for the complete data set of the hBN and SiO 2 based devices and Figure S15 for another data set of BTO based devices.
In summary, we present the integration of thin BTO layers, delaminated from the substrates where they were epitaxially grown, as ferroelectric dielectrics in single-layer MoS 2 field effect devices. Their high dielectric constant helps to screen out Coulomb scatterers leading to mobility values comparable to those obtained in devices integrating hBN dielectric, considered as an almost ideal substrate for high performance 2D-based devices in the nanomaterials community. In the devices integrating BTO layers we observe a robust hysteresis of the current vs gate voltage traces, very distinct with respect to the hysteresis observed when other dielectrics are used, which we attribute to the ferroelectric polarization switching of the BTO layer. A proof that the poling of the BTO layer is playing an important role in the hysteretic behavior is obtained from electrical characteristics at different temperatures, finding that the hysteresis nearly disappears at temperatures near the ferroelectric-to-paraelectric transition. Our results are the first step toward the fabrication of 2D-based devices with BTO based ferroelectric dielectrics and, in a more general perspective, toward the integration of a large family of transition metal oxides displaying strongly correlated physical phenomena in 2D-based devices.

■ MATERIALS AND METHODS
Epitaxial Growth of BTO Thin Films. Fifteen nanometers of LSMO and 15−50 nm of BTO thick films were grown epitaxially onto (001) STO substrates by pure oxygen (3.2 mbar) sputtering technique at high temperature (900°C). Both layers were grown sequentially without breaking vacuum. Interfaces were atomically sharp with LaO termination planes of the LSMO Manganite facing TiO 2 planes of the BTO as shown by scanning transmission electron microscopy (STEM), combined with electron energy-loss spectroscopy (EELS). This growth mode yielded out of plane ferroelectric polarization, preferentially pointing downward. 73 Release of BTO Thin Films from Their Parent Substrate. The strained LSMO/BTO heterostructure is adhered to a commercial polydimethylsiloxane film (Gel-Film WF 4 × 6.0 mil by Gel-Pack) and immersed at room temperature in a diluted solution of 0.5 mL KI (3 mol/L) + 0.5 mL HCl (37%) and 10 mL deionized H 2 O, which dissolves the LSMO in an average time of 3 days and allows the delamination of the BTO layer without damaging its properties.
Nail-Polish Deterministic Transfer of BTO. The Nitto tape with the BTO flakes is adhered to a SiO 2 /Si substrate and gently removed to transfer the BTO. The SiO 2 /Si acts as a carrier substrate followed by surface inspection by optical microscopy to find a suitable BTO flake. The flake is then picked-up with a nail-polish deterministic transfer method 77 and placed between two gold electrodes prepatterned on a SiO 2 (285 nm)/Si substrate.
Deterministic Transfer from Nitto Tape of BTO. we select the BTO flake to be transferred by optical microscopy inspection of the Nitto tape surface. The Nitto tape is then mounted into a deterministic placement setup to align the selected BTO flake between two gold electrodes prepatterned on a SiO 2 (285 nm)/Si substrate. By gently pressing the tape against the substrate and peeling off very slowly with the help of a micrometer manual actuator of the transfer setup one can transfer the BTO flake to the desired location.

Nano Letters
pubs.acs.org/NanoLett Letter Scanning Transmission Electron Microscopy. STEM-EELS characterization was carried out using a JEOL JEM-ARM 200cF aberration corrected electron microscope operated at 200 kV, equipped with a cold field emission gun and a Gatan Quantum spectrometer. BTO freestanding flakes were transferred onto a holey Si 3 N 4 membrane for STEM-EELS observation using the deterministic transfer method described above.
Piezoresponse Force Microscopy Measurements. PFM measurements were carried out using a commercial AFM system and software from Nanotec 78 operating in ambient conditions. SiO 2 /Si substrates with an e-beam evaporated film of 5 nm Ti and 15 nm Au were used for the PFM measurements. PtIr-coated commercial tips from Nanosensors (PPP-NClPt) were used and V DC and V AC voltages were applied to the tip while the sample was grounded. The drive frequency, drive amplitude (V AC ), and trigger force were The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.2c02395.
Lattice constants determination from high resolution STEM images; thickness determination of thin BTO flakes using apparent optical color; optical method to determine the thickness of BTO flakes through quantitative analysis of their optical contrast; Raman and photoluminescence spectroscopy of MoS 2 on BTO; X-ray diffraction to determine the out-of-plane lattice constant and estimate the thickness of freestanding BTO layers; comparison between the thickness determined by different techniques for the LSMO/BTO stack and the BTO film before and after releasing as a function of the growing time; local PFM amplitude and phase hysteresis curves acquired on additional transferred BTO flakes; process of laser trimming to ensure that the electrical transport occurs along the 1L-MoS 2 and the multilayer part is electrically isolated from the leads; scanning photocurrent microscopy measurement performed to demonstrate that the laser-cut process effectively disconnected the 1L-MoS 2